Method of recycling abrasive slurry

ABSTRACT

A method of recycling an abrasive slurry for recycling a slurry that: contains colloidal silica; and has been used in polishing semiconductor wafer(s) is provided. The method includes: adding a dispersant to the used slurry having been collected so as to prevent the used slurry from being gelled; irradiating ultrasound to the used slurry having been added with the dispersant so as to disperse a gelled portion and aggregated silica in the used slurry; and, by using a filter, removing a foreign substance contained in the used slurry having been irradiated with the ultrasound.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of recycling an abrasive slurry for recycling a used slurry having been used in polishing semiconductor wafer(s).

2. Description of Related Art

Polishing of semiconductor wafer(s) is generally classified into two major categories of rough polishing and finish polishing according to surface roughness to be made on the semiconductor wafer(s).

In finish polishing, where ultra-fine surface roughness is required to be made, the semiconductor wafer(s) is usually polished with an ammonia-base colloidal silica slurry having been added with a water-soluble polymer such as ethylcellulose.

The colloidal silica slurry having been used in finish polishing has conventionally been discarded because the slurry may contain metal contamination originating from components of a polishing apparatus, a giant silica solid formed by aggregation of silica in the slurry, and the like.

However, in view of environment protection, it is preferable to recycle such a wasted slurry. For this purpose, the following recycling methods of an abrasive slurry have been suggested.

For instance, a Document 1 (JP-A-2002-170793) proposes a method of reproducing a slurry, according to which coarse particles in a CMP (chemical mechanical polishing) slurry are filtrated by a filter and the filtrated slurry is condensed by such a method as centrifugation.

Alternatively, a Document 2 (JP-A-2004-63858) proposes a method of retrieving a slurry, according to which aggregated abrasive grains contained in a used slurry are crushed by ultrasound, a temperature of the slurry is adjusted and the aggregated abrasive grains are separated from non-aggregated abrasive grains.

However, the methods disclosed in the above Documents are not sufficiently applicable in retrieving and recycling the above-described colloidal silica slurry, and the following problems are pertinent to the methods.

Specifically, since ammonia contained in the colloidal silica slurry is a volatile substance, a pH value of the colloidal silica slurry may be varied by the time when the slurry is retrieved, so that the retrieved slurry may not be directly recycled.

In addition, the water-soluble polymer contained in the slurry tends to be aggregated to form a gel. Thus, even when the slurry experiences filtration by a filter, such gel-like aggregation can easily clog the filter.

Further, in finish polishing, which is the last process in manufacturing processes of semiconductor wafer(s), greater cautions are required to be paid to metal contamination.

The methods disclosed in the above Documents cannot solve all the problems although the methods may be able to solve one of the problems.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method of recycling an abrasive slurry, by which a used slurry having been used in polishing semiconductor wafer(s), particularly a slurry having been used in finish polishing, can be recycled, so that considerable reduction in slurry usage can contribute to reduction in manufacturing cost of the semiconductor wafer(s).

A method of recycling an abrasive slurry according to an aspect of the invention is for recycling a slurry containing colloidal silica, the slurry being a used slurry having been used in polishing semiconductor wafer(s), the method including:

adding a dispersant to the used slurry having been collected so as to prevent the used slurry from being gelled;

irradiating ultrasound to the used slurry having been added with the dispersant and dispersing a gelled portion and aggregated silica in the used slurry; and

removing a foreign substance contained in the used slurry having been irradiated with the ultrasound, the foreign substance being removed by a filter.

The dispersant may be any one of (1) salt, (2) polarizable molecule and (3) pH stabilizer.

-   (1) Salt: Salts formed by combining a cation selected from Li⁺, Na⁺,     K⁺, Mg²⁺, Ca²⁺ and NH₄ ⁺ with an anion selected from CO₃ ²⁻, Cl⁻,     SO₄ ²⁻, F⁻, NO³⁻, PO₄ ³⁻, CH₃COO⁻ and OH⁻ are all usable. -   (2) Polarizable Molecule: Any material containing ammonia water,     alcohols, sugars or ethers is usable. -   (3) pH stabilizer: Ammonia water, KOH and NaOH are usable.

According to the aspect of the invention, while the used slurry is prevented from being gelled by the addition to the dispersant to the used slurry, the gelled portion of the slurry and the aggregated silica are dispersed by the irradiation of ultrasound, and foreign substance(s) contained therein is removed by the filter. Thus, foreign substance(s) can be efficiently removed by the filter while an amount of silica contained in the slurry can be prevented from being reduced due to a capture of gelled and aggregated silica by the filter. With this arrangement, the used slurry can be favorably recycled.

In the above aspect of the invention, it is preferable that a pH value of the used slurry is measured and the pH value is adjusted by adding an alkali solution before the dispersant is added to the used slurry.

An example of the alkali solution for adjusting the pH value is ammonia water.

According to the aspect of the invention, by adjusting the pH value in advance, the slurry and silica can be further prevented from being aggregated.

In the above aspect of the invention, viscosity of the used slurry is preferably measured and adjusted with a supplement of a water-soluble polymer before the dispersant is added to the used slurry.

Examples of the water-soluble polymer for adjusting the viscosity are ethylcellulose and ethylene glycol.

According to the aspect of the invention, by supplementing the water-soluble polymer, the viscosity of the used slurry can be suitably adjusted. In this manner, an abrasive slurry suitable for recycling can be obtained.

In the above aspect of the invention, a temperature of the used slurry is preferably measured and adjusted with a use of a heat exchanger before the dispersant is added to the used slurry.

According to the aspect of the invention, by adjusting the temperature of the used slurry before the dispersant is added, gelled substance(s) contained in the used slurry can be dispersed therein at the optimal temperature condition.

In the above aspect of the invention, metal ion contained in the used slurry is preferably removed after the ultrasound is irradiated to the used slurry.

An exemplary method of removing the metal ion is to add a chelate agent to the used slurry.

The chelate agent may be an organic-base agent formed of aminocarboxylate. Examples of the chelate agent are EDTA (ethylenediaminetetraacetic acid), DTPA (diethylenetriaminepentaacetic acid) and NTA (nitrilotriacetic acid).

According to the aspect of the invention, the metal ion having been mixed into the slurry during polishing can be removed. Thus, when semiconductor wafer is polished with the used slurry, it is possible to prevent the semiconductor wafer(s) from being contaminated by metal ion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically shows an arrangement of a recycling apparatus according to an exemplary embodiment of the invention.

FIG. 2 is a flow chart showing steps of a recycling method according to the exemplary embodiment.

FIG. 3 is a graph showing differences in dispersion effects between dispersants.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT(S)

An exemplary embodiment of the invention will be described with reference to the attached drawings below.

FIG. 1 shows a recycling apparatus 1 for recycling an abrasive slurry according to the exemplary embodiment of the invention. The recycling apparatus 1 collects a slurry having been used in a finish-polishing machine 2, adjusts a pH value, a relative density and viscosity of the collected abrasive slurry and then removes foreign metal substance(s) therefrom so as to reproduce a slurry, thereby recycling the used slurry as an abrasive slurry to be used again in the finish-polishing machine 2.

The recycling apparatus 1 includes a multiple-stage cascade tank 3, a heat exchanger 4, a storage tank 5 and a foreign-substance filtrating filter 6.

The multi-stage cascade tank 3 serves as a precipitator that removes giant silica solids contained in the collected slurry by sedimentation separation. The inside of the tank 3 is partitioned into plural processing tanks by plural shuttering boards 31 each of which has a different height.

The plural shuttering boards 31 are arranged such that the plural shuttering boards 31 as a whole reduces its height as extending to a lateral lace 32 (i.e., where an outlet is provided) from a side to which the collected slurry is supplied. Then, when the used slurry is supplied in a first processing tank and subsequently overflows therein, an amount of the overflowing slurry flows into a processing tank contiguous to the first processing tank. This operation is sequentially repeated, and then the used slurry is finally ejected from the outlet.

In each of the processing tanks, giant silica solids, which have a higher specific gravity than the used slurry, are settled out while supernatant fluid of the used slurry, which has the lower specific gravity than the giant silica solids, flows into a processing tank contiguous thereto. With this operation being repeated, giant silica solids are separated and removed by sedimentation from the used slurry.

The heat exchanger 4, which is connected to a rear portion of the multiple-stage cascade tank 3 via a piping, is for adjusting a temperature of the used slurry by cooling the used slurry ejected from the multiple-stage cascade tank 3. The heat exchanger 4 is so arranged that a channel for the used slurry ejected from the outlet provided on a lower portion of the multiple-stage cascade tank 3 and a channel for circulation of cooling water are partitioned by a highly thermally-conductive material, thereby exchanging heat between the used slurry and the cooling water for temperature adjustment of the used slurry. It should be noted that the heat exchanger 4 may be selected from various heat exchangers, as long as the heat exchanger can exchange heat between liquid and liquid, examples of which are a plate-type heat exchanger, a double pipe-type heat exchanger, and a multitubular cylinder-type heat exchanger.

The storage tank 5, which is connected to a rear portion of the heat exchanger 4 via a piping, stores the used slurry having experienced temperature adjustment by the heat exchanger 4 and adjusts conditions of the used slurry. The storage tank 5 is provided with a thermometer 51, a viscometer 52, a hydrometer 53 and a pH meter 54 respectively for measuring temperature, viscosity, relative density and pH values of the used slurry stored in the storage tank 5.

A bottom of the storage tank 5 is additionally provided with a ultrasonic oscillator.

The ultrasonic oscillator irradiates ultrasound to the used slurry in the storage tank 5 so as to disperse gelled portions and aggregated silica in the used slurry. The ultrasound to be irradiated to the used slurry is preferably kHz-frequency ultrasound because MHz-frequency ultrasound may not be able to disperse aggregated silica due to influence of the water-soluble polymer agent.

The foreign-substance filtrating filter 6, which is connected to a rear portion of the storage tank 5 via a piping, filters the used slurry to capture foreign substances such as giant silica solids present in the used slurry. The foreign-substance filtrating filter 6 includes filters such as a depth filter and a membrane filter disposed in the channel in which the used slurry flows.

Next, operations of the above-described recycling apparatus 1 will be described based on the flow chart shown in FIG. 2.

After the used slurry is collected from the finish-polishing machine 2 (step S1), the collected used slurry is supplied to the multiple-stage cascade tank 3 of the recycling apparatus 1, and the supplied used slurry experiences sedimentation separation in each of the processing tanks thereof, so that giant silica solids are removed by sedimentation (step S2).

Then, a temperature of the used slurry from which giant silica solids have been removed is adjusted to a suitable temperature by circulating cooling water in the heat exchanger 4 (step S3), and the used slurry having experienced temperature adjustment is subsequently supplied to the storage tank 5. The temperature of the used slurry is adjusted to be in a range of approximately 20 to 30 degrees C. based on the measurement of the thermometer 51 in the storage tank 5.

Relative density, viscosity and a pH value of the used slurry supplied to the storage tank 5 are sequentially adjusted.

Relative density of the used slurry is adjusted by supplementing a stock solution of colloidal silica slurry to the storage tank 5 based on the measurement of the hydrometer 53 (step S4). Relative density of the used slurry is adjusted within a range of 1.010 to 1.020.

Viscosity of the used slurry is adjusted by supplementing a water-soluble polymer such as ethylcellulose to the storage tank 5 based on the measurement of the viscometer 52 (step S5). Viscosity of the used slurry is adjusted within a range of 0.004 to 0.01 Pa·s (values obtained by converting 4 to 10 cP).

A pH value of the used slurry is adjusted by supplementing ammonia water to the storage tank 5 based on the measurement of the pH meter 54 (step S6). A targeted pH value is roughly in a range of pH 10 to pH 11. An excessively low value of pH may reduce a polishing rate while an excessively high value of pH may dissolve silica.

When the above adjustments are finished, one of salt, polarizable molecule and pH stabilizer is added thereto as a dispersant so as to prevent aggregation of the used slurry (step S7). In addition, ultrasound is irradiated to the used slurry with the ultrasonic oscillator driven, so that gelled portions of the slurry and aggregated silica are dispersed (step S8). Examples of the dispersant are KCl, NH₄HCO₃ (examples of salt).

Finally, the used slurry having been irradiated with ultrasound is filtrated by the foreign-substance filtrating filter 6, so that foreign substances are removed (step S9). Then, a reproduced abrasive slurry is collected through a branch piping to be supplied to the finish-polishing machine 2 for use again.

EXAMPLE(S)

Next, examples of the invention will be described. It should be noted that the invention is not limited to the examples.

Experiment Example 1

After a slurry prepared by diluting a stock solution with water 20 times was used for polishing for 500 minutes, the used slurry was irradiated with ultrasound and subsequently filtrated by the foreign-substance filtrating filter 6 for removal of foreign substances. Then, a reproduced slurry was obtained.

Experiment Example 2

The same used slurry as in the experiment example 1 was added with ammonia water as a dispersant, irradiated with ultrasound and filtrated by the foreign-substance filtrating filter 6 for removal of foreign substances. Then, a reproduced slurry was obtained.

Experiment Example 3

The same used slurry as in the experiment example 1 was added with KCl water as a dispersant, irradiated with ultrasound and filtrated by the foreign-substance filtrating filter 6 for removal of foreign substances. Then, a reproduced slurry was obtained.

Experiment Example 4

A slurry was obtained by diluting a stock solution of colloidal silica slurry with water 20 times.

Experiment Example 5

The same used slurry as in the experiment example 1 was directly used as a reproduced slurry.

1. Quality of Polished Semiconductor Wafer

A semiconductor wafer having a diameter of 150 mm was polished using the slurry according to each of the experiment examples 1 to 5. Then, the quality of a surface of the polished semiconductor wafer was evaluated for each. The semiconductor wafer was polished under the following conditions: Politex™ was used as an abrasive pad; a table rotation number was 50 rpm; a head rotation number was 70 rpm; pressure of 80 g/cm² was applied; and the wafer was polished for 30 minutes.

Items of quality evaluation were a polishing rate, microroughness and the number of defects.

The polishing rate corresponds to a value after the wafer having a diameter of 150 mm was polished for 30 minutes.

Microroughness for short wavelengths is represented by an index number that is calculated in relation to a value indicated by a particle counter (SP1 manufactured by KLA-Tencor Corporation) after 30 minutes of polishing in the experiment example 4 (the value is set as 100). On the other hand, microroughness for long wavelengths is represented by an index number that is calculated in relation to a value obtained by averaging rms values of three points in the vicinity of the wafer center after 30 minutes of polishing in the experiment example 4 (the value is set as 100).

The number of defects is obtained by counting the number of defects on a polished surface of a single wafer after 30 minutes of polishing.

The results are shown in Table 1.

TABLE 1 Experiment Experiment Experiment Experiment Experiment Example 1 Example 2 Example 3 Example 4 Example 5 Polishing Rate 0.031 0.033 0.044 0.026 0.027 (μm/min) Microroughness: Haze 88 80 76 100 204 (Short wavelength) Microroughness: rms 107 100 102 100 207 (Long wavelength) Number of Defects 58 26 25 32 112 (pieces)

With respect to the polishing rate, the slurry irradiated with ultrasound as a whole exhibited enhanced polishing rates. Particularly, the slurry according to the experiment example 3, which was added with a dispersant made of KCl water, exhibited a greatly-enhanced polishing rate. On the other hand, in the slurry according to the experiment example 4, which was prepared by merely diluting the stock solution with water, the water-soluble polymer is considered to have had such a low dispersivity as to be partially aggregated on the to-be-polished surface at the time of polishing, and to have served like a protective film thereon. It is presumed that, as the consequence, the polishing rate of the slurry according to the experiment example 4 was low.

With respect to the microroughness for short wavelengths, as is understood from the above, any one of the experiment examples 1 to 3 exhibited a level of haze that is approximate to that of the experiment example 4, in which the slurry was prepared by diluting the stock solution with water. On the other hand, as is also understood from the above, the experiment example 5, in which the slurry experienced neither ultrasound irradiation nor filtering, exhibited a greatly-reduced value of haze.

With respect to the microroughness for long wavelengths, any one of the experiment examples 1 to 3 is also observed to have exhibited a level of roughness that is approximate to that of the experiment example 4. On the other hand, as is understood from the above, the experiment example 5 likewise exhibited an increased value of roughness.

With respect to the number of defects, any one of the experiment examples 1 to 3 is also observed to have exhibited the number of defects that is approximate to that of the experiment example 4. On the other hand, as is understood from the above, defects in the experiment example 5 were greatly increased.

It has been found from the above that, as shown in the experiment example 5, a polishing level (i.e., polishing rate, microroughness and the number of defects) exerted by directly applying the collected used slurry to polishing for recycling is not comparable to a polishing level exerted by a slurry prepared by diluting the stock solution as in the experiment example 4. It has been also found that ultrasound irradiation on the used slurry and addition of a dispersant to the used slurry can so greatly improve the values of the polishing level exerted by the slurry as to make the used slurry applicable as a reproduced slurry.

2. Filtration by Foreign-Substance Filtering Filter 6

The slurry according to each of the experiment examples 1 to 5 was filtrated by foreign-substance filtrating filters 6 each of which had a different filter size so as to verify to what degree of fineness in filter the slurry could pass through the filter. The results are shown in Table 2.

TABLE 2 Experiment Experiment Experiment Experiment Experiment Example 1 Example 2 Example 3 Example 4 Example 5 Filter Size: 20 μm A A A C C Filter Size: 10 μm C A A C C Filter Size: 5 μm C B A C C In Table 2: “A” means that the slurry could pass through the filter; “B” means that the slurry could partially pass through the filter; and “C” means that the slurry could not pass through the filter.

As is understood from Table 2, the slurry according to the experiment example 4 (slurry prepared by diluting the stock solution) and the slurry according to the experiment example 5 (used slurry having experienced no treatment) so considerably clogged the filters that the slurries could not pass through the filters.

The slurry according to the experiment example 1 (i.e., slurry irradiated with ultrasound) could pass through a filter having a filter size of 20 μm. However, the slurry clogged filters respectively having finer filter sizes of 10 μm and 5 μm and could not pass through the filters.

On the other hand, the slurry according to the experiment example 2 (slurry added with ammonia water and irradiated with ultrasound) and the slurry according to the experiment example 3 (slurry added with KCl water and irradiated with ultrasound) could pass through even a filter having a filter size of 5 μm. It has been found that silica solids (dried silica) of 1 to 10 μm order can be captured by filters.

Next, experiments were conducted on the slurry according to each of the experiment examples 1 to 3 in order to see how much silica solids could be captured by the foreign-substance filtrating filter 6. In the experiments, each slurry was circulated to experience filtering continuously for 300 minutes, and the number of dried silica having a size of 3 μm after the filtering was measured for evaluation. The results are shown in Table 3. In filtering each slurry, a filter having such a filter size as to be capable of favorably continuing the filtering was used. Specifically: a filter having a filter size of 20 μm was used for the slurry according to the experiment example 1; a filter having a filter size of 10 μm was used for the slurry according to the experiment example 2; and a filter having a filter size of 5 μm was used for the slurry according to the experiment example 3.

TABLE 3 Experiment Experiment Experiment Experiment Experiment Example 1 Example 2 Example 3 Example 4 Example 5 Number of 80 to 100 10 to 20 1 to 10 1000 to 6000 to Remaining Dried Silica 2000 8000 (After 300 minutes)

As is understood from Table 3, when the slurry is filtrated by the foreign-substance filtrating filter 6 after the addition of the dispersant and after the ultrasound irradiation, dried silica in the slurry can be dramatically removed. For comparison, when the number of dried silica in the slurry according to the experiment example 5 was measured, dried silica of approximately 6000 to 8000 was found present therein. It can be understood therefrom that the number of dried silica can be considerably decreased.

3. Observation of Dispersive Effects

Dispersive effect brought about by ultrasound irradiation and addition of a dispersant was checked by measuring an average particle diameter of fine silica particles in the slurry and zeta potential of the slurry. The above checking was conducted on the slurries according to the experiment examples 1, 3 to 5. In addition, in order to see a difference between dispersants, the following experiment example 6 was prepared. The zeta potential represents electrification of a surface of silica. The larger a value of the zeta potential becomes, the more favorable the dispersion is.

Experiment Example 6

The same used slurry as in the experiment example 1 was added with methanol as a dispersant and irradiated with ultrasound, so that a reproduced slurry was obtained.

The results are shown in Table 4.

TABLE 4 Experiment Experiment Experiment Experiment Experiment Example 1 Example 3 Example 4 Example 5 Example 6 Average Particle Diameter 137.5 77.2 134.5 129.2 148.3 (nm) Zeta Potential 6.82 24.24 5.86 0.93 7.02 (mV)

As compared with the slurry according to the experiment example 4 (slurry prepared by diluting the stock solution), the slurry according to the experiment example 5 (used slurry with no treatment) exhibited a greatly reduced value of the zeta potential and deteriorated slurry dispersivity. In contrast, the slurry according to the experiment example 1 (slurry irradiated with ultrasound) has been found to have restored a value of the zeta potential up to a zeta potential level of the slurry according to the experiment example 4. From the above, slurry dispersivity thereof has been found considerably enhanced.

Further, the slurry according to the experiment example 6 (slurry added with methanol and irradiated with ultrasound) has been found to have exhibited slightly better dispersivity than the slurry according to the experiment example 1.

The slurry according to the experiment example 3 (slurry added with KCl water and irradiated with ultrasound) exhibited much more increased value of the zeta potential than the slurries according the other experiment examples. From the above, it has been found that dispersivity can be considerably enhanced by adding the slurry with KCl water as a dispersant. In addition, the slurry reproduced by the method of the experiment example 3 exhibited a smaller value of the average particle diameter than the slurries according to the other experiment examples. In this respect as well, it has been found that the slurry according to the experiment example 3 is favorably applicable to polishing.

4. Difference in Dispersive Effect Due to Difference in Dispersant

Next, comparison was made to see a difference in an aggregation degree between used slurries respectively prepared by adding a different dispersant to the slurry according to the example 1. The results are shown in FIG. 3. In FIG. 3, “Ref” refers to a slurry with no addition, “KCl” refers to a slurry prepared by adding KCl thereto as a dispersant, and “NH₄HCO₃” refers to a slurry prepared by adding NH₄HCO₃ thereto as a dispersant. When a dispersant made of such salt is added to a slurry prepared by diluting with water 20 times, an optimal additive amount of the dispersant is in a range of 0.01 mol/L to 0.001 mol/L.

As is understood from FIG. 3, it has been found that, while KCl is also sufficiently effective in reducing the aggregation degree within a typical usage region of pH9.8 to 10.1, NH₄HCO₃ is effective in reducing the aggregation degree within a wider pH region beyond the typical usage region. Accordingly, NH₄HCO₃ has been found to be a considerably favorable dispersant.

The priority application Number JP 2007-217118 upon which this patent application is based is hereby incorporated by reference. 

1. A method of recycling an abrasive slurry for recycling a slurry containing colloidal silica, the slurry being a used slurry having been used in polishing semiconductor wafer(s), the method comprising: adding a dispersant to the used slurry having been collected so as to prevent the used slurry from being gelled; irradiating ultrasound to the used slurry having been added with the dispersant so as to disperse a gelled portion and aggregated silica in the used slurry; and removing a foreign substance contained in the used slurry having been irradiated with the ultrasound, the foreign substance being removed by a filter.
 2. The method of recycling an abrasive slurry according to claim 1, wherein a pH value of the used slurry is measured and the pH value is adjusted by adding an alkali solution before the dispersant is added to the used slurry.
 3. The method of recycling an abrasive slurry according to claim 1, wherein viscosity of the used slurry is measured and the viscosity is adjusted by adding a water-soluble polymer before the dispersant is added to the used slurry.
 4. The method of recycling an abrasive slurry according to claim 1, wherein a temperature of the used slurry is measured and the temperature is adjusted by using a heat exchanger before the dispersant is added to the used slurry.
 5. The method of recycling an abrasive slurry according to claim 1, wherein metal ion contained in the used slurry is removed after the ultrasound is irradiated to the used slurry.
 6. The method of recycling an abrasive slurry according to claim 5, wherein the metal ion contained in the used slurry is removed by adding a chelate agent to the used slurry. 